1. Field of the Invention
The present invention relates to bit rate conversion of a video transcoder and, more particularly, to an apparatus and method for adaptively controlling logarithmic R-Q model-based bit rate.
2. Description of the Background Art
With the advent of a video compression technology, digital video signal can be transmitted through diverse types of channels including the Internet, a radio network and an ATM network. Therefore, a future video service should take is a heterogeneous video coding standard, as well as a heterogeneous network environment, into account.
Up to now, MPEG-1 has been used as a general video coding standard for a digital storage medium and CD-ROM application and numerous video contents are created to an MPEG-1 bit stream. As video transmission at a low bit rate such as a moving environment is in increasingly demand, an MPEG-4 has been introduced as one of main stream technique resistant to a channel error.
Having a bit rate of about 1.5 Mbit, the MPEG-1 is not suitable for a radio environment which requests a low bit rate. In comparison, the MPEG-4 has a high compression rate and adopts various error-resistance techniques, so that it is suitable for a radio environment with a high error rate and low channel band width.
Therefore, by transcoding the MPEG-1 video contents stored in a server to the MPEG-4 with the low bit rate, various video contents can be effectively transmitted from the server to a mobile terminal having an MPEG-4 decoder through a radio environment. As a related substantial application field, a VOD (Video on Demand) or a streaming video service allows the mobile terminal with the MPEG-4 to receive MPEG-1 video contents which has been previously compressed and stored.
In order to provide such service, the high bit rate of the MPEG-1 which has been previously compressed and stored should be converted into an MPEG-4 bit stream of a low bit rate as requested by the mobile terminal with the MPEG-4, for which a video transcoder is proposed to perform the conversion operation.
FIG. 1 illustrates a structure of a conventional video transcoder for the bit rate conversion.
With reference to FIG. 1, the conventional video transcoder includes a variable length decoder (VLD) 10, an inverse quantizer (IQ1) 12, an adder 14, a quantizer 16, a variable length coder (VLC) 18, an inverse quantizer (IQ2) 20, an adder 22, a frame memory 24, a motion compensator 26, a bit rate controller 28 and an MPEG-4 syntax generator 30.
The VLD 10 decodes an MPEG-1 bit stream with a bit rate (R1), and outputs a quantization parameter (QP) and a DCT coefficient.
The inverse quantizer 12 inversely quantizes the DCT coefficient to a quantization parameter (QP) and outputs it to the adder 14. Then, the adder 14 adds the motion compensation value at the DCT domain outputted from the motion compensator 26 and the DCT coefficient outputted from the inverse quantizer 12.
The quantizer 16 quantizes the DCT coefficient outputted from the adder 14 with the quantization parameter (QP) outputted from the bit rate controller 28, and outputs it to the VLC 18. Then, the VLC 18 codes the quantized DCT coefficient with the MPEG-4 syntax outputted from the syntax generator 28 and outputs an MPEG-4 bit stream with a bit rate (R2).
At this time, the motion compensator 26 generates a motion compensation value at the DCT domain by using the motion vector (MV) outputted from the VLD 10 and a reference DCT frame stored in the frame memory 24, and the adder 22 subtracts the outputs of the inverse quantizers 12 and 20 and adds the corresponding subtraction value to the motion compensation value to generate a reference frame to be stored in the frame memory 24. The MPEG-4 syntax generator 30 generates an MPEG-4 syntax from the MPEG-1 bit stream.
In this manner, the video transcoder transcodes the MPEG-1 bit stream with the high bit rate (R1) to the MPEG-4 bit stream with the low bit rate (R2), and a target bit rate of the MPEG-4 is performed under the control of the bit rate controller 28.
The bit rate controller 28 receives the quantization parameter (QP) of the MPEG-1 which has been decoded in the VLD 10 and outputs a quantization parameter for controlling the bit rate of the MPEG-4 to the quantizer 16.
The up-to-date method for controlling the bit rate of the MPEG-4 by the bit rate controller of the video transcoder is ‘a new requantization method for MPEG-1 to MPEG-4 transcoder’ disclosed in pages 13–16 of an article presented by S. C. Heo, K. D. Seo, K. C. Roh and J. K. Kim in the IEEE ICME (IEEE Int. Conf. On Multimedia and Expo 2001) held in Tokyo.
In the above article, two steps are formed to control the bit rate of MPEG-4. First step is determining a suitable quantization parameter on the basis of an existing general R-Q (rate-quantization) model, and the second step is finally obtaining a quantization parameter of MPEG-4 in consideration of difference between a compression efficiency and quantization method of MPEG-1 and those of MPEG-4.
FIG. 2 is a block diagram of a bit rate controller for controlling a bit rate of MPEG-4 in accordance with the conventional art.
With reference to FIG. 2, the conventional bit rate controller includes an R-Q modeling unit 20 and a quantization parameter determining unit 22.
The R-Q modeling unit 20 calculates a target quantization parameter [Q1T(n,m)] of MPEG-1 by adopting an R-Q model to the inputted quantization parameter [Q1(n,m)] of MPEG-1. In this case, ‘n’ indicates an order of pictures and ‘m’ indicates an index representing the order of macro blocks in the picture. The R-Q (rate-quantization) model has been proposed by Ding in IEEE Trans. On circuits and systems for video technology, February 1997.
The quantization parameter determining unit 22 receives the target quantization parameter [Q1T(n,m)] and calculates a quantization parameter [Q2(n,m)] of MPEG-4 to obtain a bit rate of MPEG-4 in consideration of the difference in the coding efficiency between MPEG-1 and MPEG-4 and coding and the difference of the coding methods of the quantization parameter between two coding standards.
That is, the coding efficiency of MPEG-4 is better than that of MPEG-1 by about 10–20%. Thus, in order to reflect the coding efficiency, β parameter has been introduced to the quantization parameter determining unit to reflect the coding efficiency. In addition, in case of MPEG-4, in order to prevent a rapid change of picture quality, a changeable value of the quantization parameter between neighboring macro blocks is limited to ±2, while there is no such limitation in MPEG-1.
Thus, in order to resolve such difference, the quantization parameter determining unit 22 uses an gradual approach function (GAF).
The quantization parameter determining unit 22 calculates a quantization parameter [Q2(n,m)] for MPEG-4 by adopting the β parameter and the GAF to the target quantization parameter [Q1T(n,m)]. In addition, the quantization parameter determining unit 22 detects α(n) indicating a difference between a target bit rate per 1 frame and an actually generated bit count and feed it back to the R-Q modeling unit 20. Then, the R-Q modeling unit 20 increases or decreases bit count transmission as much as α(n) in the next frame.
Accordingly, the quantizer 16 quantizes the DCT coefficient outputted from the adder 14 with the quantization parameter [Q2(n,m)], thereby obtaining a target bit rate of MPEG-4.
As mentioned above, the bit rate controller obtains the target quantization parameter [Q1T(n,m)] by adopting the R-Q model to the quantization parameter [Q1(n,m)] of MPEG-1. However, the R-Q modeling unit of the bit rate controller uses only the fixed R-Q model without considering characteristics of an input image.
That is, the conventional bit rate controller has a problem that it uses a fixed model that is not changeable regardless of an image for the R-Q model, a core of a bit rate control algorithm, it can not control accurately the bit rate. Because of this problem, an accurate bit rate is not assigned to each picture, resulting in that a picture quality is degraded or picture quality between pictures is provided a method for adaptively controlling a bit rate of a video transcoder including the steps of: converting a bit stream of MPET-1 into a DCT coefficient; generating a quantization parameter of MPEG-4 by reflecting a difference of a logarithmic R-Q model and a coding efficiency for a quantization parameter of MPEG-1; quantizing the DCT coefficient with the generated quantization parameter of MPEG-4 and outputting an MPEG-4 bit stream.
In the method for adaptively controlling a bit rate of a video transcoder of the present invention, the step of generating a quantization parameter of MPEG-4 includes: setting an initial value of a control parameter; determining a quantization parameter of a target MPEG-1 by adopting a logarithmic R-Q model to a quantization parameter of MPEG-1; adjusting the quantization parameter of the target MPEG-1 by using a correction parameter of a coding efficiency between MPEG-1 and MPEG-4; and outputting the quantization parameter of MPEG-4 by adopting a gradual approach function to the adjusted quantization parameter; and updating a control parameter when one picture is completely processed.
To achieve at least these advantages in whole or in parts, there is further provided a method for adaptively controlling a bit rate of a video transcoder including the steps of: setting an initial value of a control parameter; determining a quantization parameter of a target MPEG-1 by adopting a logarithmic R-Q model to a quantization parameter of an input MPEG-1; adjusting the quantization parameter of the target MPEG-1 by using a parameter indicating a difference of a coding efficiency between MPEG-1 and MPEG-4; calculating a quantization parameter of an output MPEG-4 by adopting a gradual approach function to the adjusted quantization parameter of the target MPEG-1; calculating a difference between a target bit count for each picture and an actually generated bit count and feeding back the difference to the quantization parameter determining step; and updating a control parameter when one picture is completely processed.
To achieve at least these advantages in whole or in parts, there is further provided an apparatus for adaptively controlling a bit rate of a video transcoder including: a logarithmic R-Q modeling unit for outputting a quantization parameter of a target MPEG-1 by adopting a logarithmic R-Q model to a quantization parameter of an input MPEG-1; a quantization parameter adjusting unit for adjusting a quantization parameter of the target MPEG-1 with a correction parameter of a coding efficiency; quantization parameter determining unit for outputting a quantization parameter of an output MPEG-4 by adopting an gradual approach function to the adjusted quantization parameter of the target MPEG-1; and bit count computing unit for calculating a bit count difference between the nth picture of the target MPEG-1 and that of the output MPEG-4, and feeding back the bit count difference to the logarithmic R-Q modeling unit.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.